Comprehensive study of carrier dynamics in gradient hydrogenated NdNiO3 using broadband ultrafast transient reflectivity spectroscopy

Hydrogenation of perovskite rare-earth nickelates induces giant lattice distortions and enhanced tilting/rotation of NiO6 octahedra, enabling a room-temperature metal–insulator transition. However, unraveling the couplings between ion, electron, and lattice degrees of freedom introduced by hydrogenation remains a significant challenge. In this work, we investigate the electronic dynamics of gradient hydrogen-doped correlated NdNiO3 (NNO) film using broadband femtosecond transient reflectivity spectroscopy. It is found that the incorporation of hydrogen suppresses the phonon-polariton beats in pristine NNO while a biexponential decay process appears in lightly hydrogen-doped NNO. The additional relaxation channels introduced by hydrogen compete with the bottleneck effect mediated by bandgap opening, which would affect the ultrafast electronic dynamics. Meanwhile, the markedly accelerated slow relaxation in heavily hydrogen-doped NNO is consistent with nonradiative recombination mediated by impurity defect states. These results reveal that hydrogen incorporation not only modifies the equilibrium electronic structure of NdNiO3 but also acts as an ionic control knob for reshaping nonequilibrium carrier relaxation pathways. This work extends hydrogen-controlled nickelate physics from static phase modulation to ultrafast carrier dynamics, highlighting hydrogenation as a powerful strategy for engineering relaxation timescales and recovery pathways in correlated-oxide iontronic devices.